Method of producing an absorbent structure having improved strength properties

ABSTRACT

A method of producing an absorbent structure that includes hydrophilic fibres and particles of superabsorbent material, comprising forming an air-laid structure from fibres and superabsorbent particles. At least a part of the air-laid structure is moistened to a moisture content in the moistened region of at least 15% calculated on the total weight of the structure within the moistened region, whereafter the structure is dried to a moisture content of at most 12%. The moisture treatment improves the tensile strength of the absorbent structure. Liquid dispersion in the structure is also improved.

FIELD OF INVENTION

The present invention relates to a method of producing an air-laidabsorbent structure that includes hydrophilic fibres and particles ofsuperabsorbent material. Such absorbent structures are used in sanitaryarticles, such as diapers, incontinence guards, sanitary napkins andlike articles.

DESCRIPTION OF THE BACKGROUND ART

An absorbent structure in a sanitary article of the aforesaid kindintended for one-time use only is typically comprised of one or morelayers of hydrophilic fibres, normally cellulose fluff pulp. Thestructure will also often include so-called superabsorbents, which arepolymers that can absorb many times their own weight in water or bodyliquid. Such an absorbent structure is flexible and comfortable to wear,and also has a high absorption capacity.

One drawback of these known absorbent structures is their relatively lowstrength properties, particularly tensile strength. which can sometimescause problems in the various steps of manufacturing the sanitaryarticle in question and also during the use of said article. Variousattempts have been made to enhance the strength and the structuralcoherency of such absorbent structures. among other things by mixing-inthermoplastic synthetic fibres with subsequent heating of the absorbentstructure, c.f. for instance U.S. Pat. No. 4,590,114. The thermoplasticfibres are therewith melted and contribute towards a more coherentstructure that has improved strength properties. The drawback with thissolution is the relatively high price of the thermoplastic fibres andthe negative influence that this so-called thermobonding has on theabsorption properties.

A method of producing a wet-laid absorbent structure of relatively hightensile strength is disclosed in U.S. Pat. No. 4,551,142. for instance.This method involves providing an aqueous dispersion of cellulose fibresand superabsorbent particles, formation of this dispersion into awet-laid sheet which is then dewatered, dried and compressed to thedesired density. The procedure and equipment required in this processare completely different to those normally applied in the air-layingmethod, in which cellulose pulp in bale or sheet form is dry-shredded toform so-called fluff pulp and air-laid to form a pulp mat together withsuperabsorbent particles.

U.S. Pat. No. 5,516,569 discloses that an absorbent composite containing40 to 85% superabsorbent particles is wetted to a moisture content ofbetween 15 to 30% in order to bind the superabsorbent particles in thecomposite.

OBJECTS AND MOST IMPORTANT FEATURES OF THE INVENTION

The object of the present invention is to provide a method of producingan absorbent structure formed in accordance with an air-laying methodand having considerably improved strength properties. This object isachieved in accordance with the invention by moistening the air-laidstructure to a moisture content of at least 15%, preferably at least20%. and more preferably at least 35%, calculated on the total weight ofthe structure, and thereafter drying the structure to a moisture contentof at most 12%, preferably at most 10%.

When the absorbent structure is moistened or wetted to a high moisturecontent, water may be pressed from the structure prior to drying thesame.

The structure is preferably moistened with distilled water or de-ionizedwater.

The inventive method is flexible, by virtue of the fact that it is onlynecessary to moisten or wet certain parts of the structure in order toobtain better strength. For instance, the structure can be moistened ina suitable moistening pattern, e.g. in a strip pattern or networkpattern. In the case of profile structures where different parts of thestructure have mutually different weights per unit area, it may besuitable to moisten only those parts that have the lowest weight perunit area and therewith the lowest strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron microscope photograph of a fluff pulp structurethat includes superabsorbent particles.

FIG. 2 is a corresponding photograph of a corresponding structuretreated in accordance with the invention.

DESCRIPTION OF THE INVENTION

The absorbent structure is formed by a conventional air-laying method,see, for instance, EP 0 192 042, according to which cellulose pulp isdry-shredded to cellulose fluff pulp and a mixture of this pulp andsuperabsorbent particles is air-laid to form a web, or is air-laid inmoulds on a so-called mat-forming, wheel, for forming the absorbentstructure. The superabsorbent particles may alternatively be applied asa layer between pulp layers, instead of being admixed with the pulpfibres. It is also possible to form two or more layers one on top of theother. These layers may contain mutually different percentages ofsuperabsorbent, different types of pulp fibres and/or superabsorbent, soas to impart desired properties to the absorbent structure, such asliquid acquisition, liquid dispersion and liquid storage properties. Theconstruction of such absorbent structures is well known in the art andwill not therefore be described in detail here.

The cellulose fibres may be comprised of chemical pulp, mechanical pulp,thermomechanical or chemithermomechanical pulp (CTMP). Fibres ofchemically cross-linked cellulose may also be used. Other types ofhydrophilic fibres are regenerated cellulose (viscose), polyester andhydrophobic fibres that have been treated with a hydrophilizing agent.

Various different types of superabsorbents may be used. A common featureof these superabsorbents is that they are polymers which are capable ofabsorbing and binding water or body liquid in quantities correspondingto many times their own weight. Examples of polymers that are used tothis end are polyacrylates, alginates, polysaccharides, such ascellulose derivatives, starch derivatives, etc., and copolymers andgraft polymers thereof.

The superabsorbent particles will preferably be present in an amountcorresponding to at least 3% calculated on the total weight of thestructure before moistening, preferably at least 5% and at most 80%,preferably at most 70% and more preferably at most 35% superabsorbentparticles.

The air-laid absorbent structure is moistened with water in themoistened region to a moisture content of at least 15%, preferably atleast 20% and more preferably at least 35%. calculated on the totalweight of the structure. The term moistening as used here also includeswetting the structure with water to the saturation concentration in thewetted regions. The water used will preferably be distilled water orde-ionized water, which has the least possible affect on the absorbencyof the superabsorbents subsequent to moistening and drying thestructure.

The entire structure may be moistened, or solely parts of saidstructure. For instance, the structure may be moistened inlongitudinally extending or transversely extending strips, in a networkpattern or in some other pattern judged to be suitable in context. Inthe case of absorbent structures that have a profiled appearance, wheredifferent parts of the structure have different weights per unit areasit may be suitable to moisten only those parts that have the lowestweight per unit area and are therewith the weakest.

The structure is thereafter dried in some suitable way to a moisturecontent of at most 12%, preferably at most 10%, calculated on the totalweight of the structure. Part of the liquid may be pressed mechanicallyfrom the structure prior to drying said structure. This may beappropriate when an excessive amount Of liquid has been used to moistenthe structure. The absorbent structure will preferably remain moist overa time period of at least one minute. This is to enable thesuperabsorbent particles to absorb liquid and swell. Swelling of thesuperabsorbent particles results in a certain adhesion betweensuperabsorbent and pulp fibres, and it can be said that the swollensuperabsorbents function as a binder in the fibre structure thatincreases the strength of said structure. As will be evident from FIGS.1 and 2, the superabsorbent grains do not retain their original particleshape after treatment, but instead “creep” along and between the fibres.

After being dried, the structure can be compressed to the densitydesired, preferably between 0.06 and 0.3 g/cm³. The tensile strength ofthe thus produced absorbent structure will be much higher than thetensile strength of a corresponding structure that has not beensubjected to moisture treatment.

The absorbent structure can be incorporated as an absorbent body in anabsorbent article, such as a diaper, an incontinence guard, a sanitarynapkin and like article. The absorbent structure is herewith typicallyenclosed between a liquid-permeable top sheet, which suitably comprisesa nonwoven material or a perforated plastic film, and aliquid-impermeable backing sheet, normally comprised of plastic film,e.g. polyethylene film. In certain cases, a liquid-acquisition materialin the form of porous wadding or nonwoven material may be arrangedbetween the top sheet material and absorbent body. The absorbent articlemay also be provided with elastic devices that shape and adapt thearticle to the wearer's body.

With the intention of discovering how such moisture treatment affectsthe properties of the absorbent structure, tests were carried out on thefollowing pulp/superabsorbent mixtures:

A) STORA CTMP+7% superabsorbent IM 7100 from Hoechst.

B) Korsnäs Vigorfluff A (chemical pulp)+30% superabsorbent IM 7100.

C) Weyerhaeuser NB 416 (chemical pulp)+30% superabsorbent IM 7100.

D) Chemical pulp (200 g/m²)+60% superabsorbent IM 7100.

Sample bodies measuring 50×100 mm and containing a mixture of pulpfibres and superabsorbent particles were punched-out. The samples werecompressed to a bulk value of 6 cm³/g (density 0.17 g/cm³) andthereafter conditioned for 24 h. The tensile strength of the samples wasmeasured in an Instron apparatus. The sample bodies were then wettedwith de-ionized water, substantially to saturation, and thereafter driedat 50° C. over 10-14 hours. The samples were conditioned and the bulkadjusted to 6 cm³/g (density 0.17 g/cm³). The tensile strength of thesamples was measured in the same way as that described above. Theresults obtained are given in the following Tables.

TABLE 1 Tensile strength of samples before and after treatment. A be- AB be- B C be- C D be- D Sample fore after fore after fore after foreafter Tensile 0.88 8.32 5.75 16.98 5.76 16.14 1.22 9.38 strength (N/50mm)

The measurement values constitute the mean value of seven measuringprocesses.

It will be seen that the tensile strength was increased manifold as aresult of the treatment.

When the chemical pulp has a very high content of superabsorbentparticles as in Example D, the tensile strength is low. The tensilestrength increases with the treatment. However, the strength is stilllow in comparison with the strength of the other treated chemical pulpscontaining only 30% superabsorbent.

With the intention of discovering how the absorbent properties wereaffected by the treatment, measurements were carried out with respect toabsorbency and dispersion distance. The measurements were carried out inaccordance with the following methods:

The samples were placed in the test equipment with one part horizontaland one part inclined at 60°, and allowed to absorb liquid in accordancewith the self-suction principle from an “infinite” source. After a givenperiod of time (the absorption time), the extent to which the liquid haddispersed was determined (the dispersion distance). The amount of liquidthat had been absorbed (the absorption) was also measured with the aidof scales.

The sample was then placed in liquid and allowed to absorb freely,whereafter the sample was placed on a grating and loaded with a weightof 7.0 kg for a period of 5 min. The sample was weighed to give the freeabsorption under load. The measurement values thus obtained can then beused to calculate a measurement of the sample utilization potential(utilization degree) in accordance with:$\frac{{{Abs}.\quad {- \quad {Sample}}}\quad {Weight}}{{Free}\quad {{abs}.\quad {- \quad {Sample}}}\quad {weight}} \times 100\quad (\%)$

The results are given in Table 2 below.

TABLE 2 Absorption properties before and after treatment. A be- A B be-B C be- C D be- D Sample fore after fore after fore after fore afterSample weight 16 15 37 37 37 37 24.5 24.4 (g) Abs. (g) 125 127 426 432378 419 397 407 Utilize degree 38 45 53 60 47 60 57 56 (%) Free abs.after 302 259 775 690 764 675 681 714 loading (g) Abs. time 60 60 120120 120 120 150 150 (min) Disp distance 25.5 34 28.5 34 26.5 34.5 25.726.3 (cm)

The measurement values constitute the mean measurement of threemeasuring processes.

It will be seen from these measurements that the dispersion distanceincreased after treatment, which also gives a higher degree ofutilization. On the other hand, a slightly lower free absorption wasobtained after loading the sample, which can be assumed to be becausethe ability of the superabsorbent to take-up liquid has been impairedafter treatment. This is compensated for, however, by the improveddispersion distance, and hence absorption was roughly the same beforeand after treatment.

When the pulp has a high content of superabsorbent, above about 50%, thedispersion distance will not increase as much as for the pulps having alower content of superabsorbent. Neither will the utilization potentialincrease as much. This is demonstrated by Example D where both thedispersion distance and the utilization potential remain essentiallyunchanged by the treatment.

Thus, a superabsorbent content below 60% is preferred, especially acontent below 50%, a content below 40 or 35% being most preferred.

What is claimed is:
 1. A method of producing an absorbent structure thatincludes hydrophilic fibres and particles of superabsorbent material,comprising forming an air-laid structure of fibres and superabsorbentparticles, moistening at least part of said air-laid structure to amoisture content in the moistened region of at least 15%, calculated onthe total weight of the structure within the moistened region, andthereafter drying the structure to a moisture content of at most 12%,the structure containing 3-35% superabsorbent particles, calculated onthe total weight of the structure before moistening.
 2. A methodaccording to claim 1, where the absorbent structure contains at least 5%superabsorbent particles, calculated on the total weight of thestructure before moistening.
 3. A method according to claim 1,comprising moistening to a moisture content in the moistened region ofat least 20%, calculated on the total weight of the structure within themoistened region.
 4. A method according to claim 3, comprisingmoistening to a moisture content in the moistened region of at least35%, calculated on the total weight of the structure within themoistened region.
 5. A method according to claim 1, comprising dryingthe structure to a moisture content of at most 10%.
 6. A methodaccording to claim 1, comprising moistening the structure with distilledor de-ionized water.
 7. A method according to claim 1, comprisingpressing water from the moistened structure, prior to drying saidstructure.
 8. A method according to claim 1, comprising moistening onlycertain parts of the structure.
 9. A method according to claim 8,comprising moistening the structure in such a pattern or within suchparts of the structure that the total tensile strength of said structureis enhanced.
 10. An absorbent article, including an absorbent structureobtained by the method according to claim
 1. 11. A method according toclaim 2, comprising moistening to a moisture content in the moistenedregion of at least 20%, calculated on the total weight of the structurewithin the moistened region.
 12. A method according to claim 2,comprising drying the structure to a moisture content of at most 10%.13. A method according to claim 2, comprising moistening the structurewith distilled or de-ionized water.
 14. A method according to claim 2,comprising pressing water from the moistened structure, prior to dryingsaid structure.
 15. A method according to claim 2, comprising moisteningonly certain parts of the structure.
 16. An absorbent article, includingan absorbent structure obtained by the method according to claim
 2. 17.An absorbent article according to claim 10, wherein the absorbentarticle is a diaper, incontinence guard, or a sanitary napkin.
 18. Anabsorbent article according to claim 16, wherein the absorbent articleis a diaper, incontinence guard, or a sanitary napkin.